Cannot remove double vertices
Merge by distance should fix your problem, but you need to select the entire model. If it still doesn't work try increasing the merge distance.

Try identifying of which vertices you have duplicates by using the box select tool on one of them and paying attention to the bottom left of the window. If it says "Verts: 2/13" you press "alt + m" and click on "at center". Repeat this until all duplicates are gone.